The covering power of a silver halide emulsion is a matter of great concern for one engaged in preparation of silver halide emulsions. This is because if an emulsion having a high covering power is used, the amount of silver necessary to obtain a desired optical density can be decreased.
U.S. Pat. Nos. 2,996,382, 3,178,282, 3,397,987, and 3,607,278, British Pat. No. 1,426,277, etc., describe that the covering power of silver halide can be greatly increased by mixing a surface light-sensitive emulsion having a high silver iodide content and an internally fogged emulsion containing small grains.
It is also known that to increase the covering power of a silver halide emulsion, various additives can be added to the emulsion. The majority of additives capable of increasing the covering power are synthetic polymers or saccharides. Typical examples of such synthetic polymers are polyacrylamides such as those described in U.S. Pat. Nos. 3,271,158 and 3,514,289. Typical examples of such saccharides are dextran, as described in U.S. Pat. Nos. 3,063,838 and 3,272,631.
These additives, however, have a disadvantage in that if they are used in a sufficient amount to achieve the above object, the resulting light-sensitive material is excessively increased in contrast, because development activity is markedly increased, thereby resulting in a reduction of graininess.
Japanese Patent Application (OPI) No. 215647/83 (the term "OPI" as used herein refers to a published unexamined Japanese patent application) discloses a method for controlling gradation by separating surface light-sensitive layers and introducing internally fogged fine grains only in the low-sensitive layer. In accordance with this method, however, it is difficult to control the developing properties of areas composed of surface light-sensitive emulsion grains on the low-sensitive layer, and graininess of high density areas cannot be improved.
Moreover, since a high iodine content emulsion is used, if a light-sensitive material is continuously processed, iodine ions accumulate in the processing solution, and the amount of the iodine ion accumulated in the processing solution is significantly larger than that in the case of using the usual low iodine content light-sensitive material. Thus, if the number of light-sensitive materials processed exceeds a certain value, an increase in fog is caused.
This formation of fog is readily encountered in using developers in which the amount of iodine ions released from the light-sensitive material is large.
In order to overcome the above defects, particularly to improve stability during the development, several methods have been proposed. For example, Japanese Patent Publication No. 25691/77 and Research Disclosure, RD No. 18431, page 434 (Aug., 1979) describe the yellow fog-preventing effect due to addition of a nitron salt (1,4-diphenyl-3,5-endoanilino-4,5-dihydro-1,2,4-triazole). Japanese Patent Application (OPI) Nos. 87322/85, 117240/85, and 122936/85 disclose an improvement in dependency on halogen (e.g., iodine) ions contained in the processing solution through addition of a mesoionic triazolimine compound other than the nitron.
These methods, however, are not very effective, and rather cause serious desensitization. Thus, they are not satisfactory for practical use.